Human eye

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Human eye
Eye-diagram no circles border.svg
Details
Identifiers
Latin Oculi Hominum
Greek ἀνθρώπινος ὀφθαλμός
MeSH D005123
TA A01.1.00.007
A15.2.00.001
FMA 54448
Anatomical terminology

The human eye is an organ which reacts to light and pressure. As a sense organ, the mammalian eye allows vision. Human eyes help to provide a three dimensional, moving image, normally coloured in daylight. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth. The human eye can differentiate between about 10 million colors [1] and is possibly capable of detecting a single photon. [2]

Organ (anatomy) collection of tissues joined as a structural unit that serves a common function

Organs are groups of tissues with similar functions. Plant and animal life relies on many organs that coexist in organ systems.

Light electromagnetic radiation in or near visible spectrum

Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is the visible spectrum that is visible to the human eye and is responsible for the sense of sight. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), or 4.00 × 10−7 to 7.00 × 10−7 m, between the infrared and the ultraviolet. This wavelength means a frequency range of roughly 430–750 terahertz (THz).

Mammalian eye

Mammals normally have a pair of eyes. Although mammalian vision is not so excellent as bird vision, it is at least dichromatic for most of mammalian species, with certain families possessing a trichromatic color perception.

Contents

Similar to the eyes of other mammals, the human eye's non-image-forming photosensitive ganglion cells in the retina receive light signals which affect adjustment of the size of the pupil, regulation and suppression of the hormone melatonin and entrainment of the body clock. [3]

Melatonin hormone that is produced by the pineal gland in animals and regulates sleep and wakefulness

Melatonin is a hormone that regulates sleep-wake cycles. It is primarily released by the pineal gland. As a supplement, it is often used for the short-term treatment of trouble sleeping such as from jet lag or shift work. Evidence of benefit, however, is unclear. One review found onset of sleep occurred 6 minutes faster with use but found no change in total time asleep. It may work as well as the medication ramelteon. It is typically taken by mouth.

In the study of chronobiology, entrainment occurs when rhythmic physiological or behavioral events match their period to that of an environmental oscillation. It is ultimately the interaction between circadian rhythms and the environment. A central example is the entrainment of circadian rhythms to the daily light–dark cycle, which ultimately is determined by the Earth's rotation. Exposure to certain environmental stimuli will cue a phase shift, and abrupt change in the timing of the rhythm. Entrainment helps organisms maintain an adaptive phase relationship with the environment as well as prevent drifting of a free running rhythm. This stable phase relationship achieved is thought to be the main function of entrainment.

Circadian rhythm any biological process in an organism that recurs with a regularity of approximately 24 hours.

A circadian rhythm is any biological process that displays an endogenous, entrainable oscillation of about 24 hours. These 24-hour rhythms are driven by a circadian clock, and they have been widely observed in plants, animals, fungi, and cyanobacteria.

Structure

Blood vessels can be seen within the sclera, as well as a strong limbal ring around the iris. Human eye with blood vessels.jpg
Blood vessels can be seen within the sclera, as well as a strong limbal ring around the iris.
The outer parts of the eye. Gray892.png
The outer parts of the eye.

The eye is not shaped like a perfect sphere, rather it is a fused two-piece unit, composed of the anterior segment and the posterior segment. The anterior segment is made up of the cornea, iris and lens. The cornea is transparent and more curved, and is linked to the larger posterior segment, composed of the vitreous, retina, choroid and the outer white shell called the sclera. The cornea is typically about 11.5 mm (0.3 in) in diameter, and 1/2 mm (500 μm) in thickness near its center. The posterior chamber constitutes the remaining five-sixths; its diameter is typically about 24 mm. The cornea and sclera are connected by an area termed the limbus. The iris is the pigmented circular structure concentrically surrounding the center of the eye, the pupil, which appears to be black. The size of the pupil, which controls the amount of light entering the eye, is adjusted by the iris' dilator and sphincter muscles.

Iris dilator muscle

The iris dilator muscle, is a smooth muscle of the eye, running radially in the iris and therefore fit as a dilator. The pupillary dilator consists of a spokelike arrangement of modified contractile cells called myoepithelial cells. These cells are stimulated by the sympathetic nervous system. When stimulated, the cells contract, widening the pupil and allowing for more light to pass through the eye.

Iris sphincter muscle

The iris sphincter muscle is a muscle in the part of the eye called the iris. It encircles the pupil of the iris, appropriate to its function as a constrictor of the pupil.

Light energy enters the eye through the cornea, through the pupil and then through the lens. The lens shape is changed for near focus (accommodation) and is controlled by the ciliary muscle. Photons of light falling on the light-sensitive cells of the retina (photoreceptor cones and rods) are converted into electrical signals that are transmitted to the brain by the optic nerve and interpreted as sight and vision.

Photoreceptor cell specialized type of cell found in the retina that is capable of visual phototransduction

A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light into signals that can stimulate biological processes. To be more specific, photoreceptor proteins in the cell absorb photons, triggering a change in the cell's membrane potential.

Size

Dimensions typically differ among adults by only one or two millimetres, remarkably consistent across different ethnicities. The vertical measure, generally less than the horizontal, is about 24 mm. The transverse size of a human adult eye is approximately 24.2 mm and the sagittal size is  23.7 mm with no significant difference between sexes and age groups. Strong correlation has been found between the transverse diameter and the width of the orbit (r = 0.88). [4] The typical adult eye has an anterior to posterior diameter of 24 millimetres, a volume of six cubic centimetres (0.4 cu. in.), [5] and a mass of 7.5 grams (weight of 0.25 oz.).[ citation needed ].

The eyeball grows rapidly, increasing from about 16–17 millimetres (about 0.65 inch) at birth to 22.5–23 mm (approx. 0.89 in) by three years of age. By age 12, the eye attains its full size.

Components

Schematic diagram of the human eye. It shows a horizontal section through the right eye. Schematic diagram of the human eye en.svg
Schematic diagram of the human eye. It shows a horizontal section through the right eye.

The eye is made up of three coats, or layers, enclosing various anatomical structures. The outermost layer, known as the fibrous tunic, is composed of the cornea and sclera. The middle layer, known as the vascular tunic or uvea, consists of the choroid, ciliary body, pigmented epithelium and iris. The innermost is the retina, which gets its oxygenation from the blood vessels of the choroid (posteriorly) as well as the retinal vessels (anteriorly).

Fibrous tunic of eyeball

The sclera and cornea form the fibrous tunic of the bulb of the eye; the sclera is opaque, and constitutes the posterior five-sixths of the tunic; the cornea is transparent, and forms the anterior sixth.

Cornea part of the eye

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. The cornea, with the anterior chamber and lens, refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43 dioptres. The cornea can be reshaped by surgical procedures such as LASIK.

Sclera

The sclera, also known as the white of the eye, is the opaque, fibrous, protective, outer layer of the human eye containing mainly collagen and some elastic fiber. In humans, the whole sclera is white, contrasting with the coloured iris, but in other mammals the visible part of the sclera matches the colour of the iris, so the white part does not normally show. In the development of the embryo, the sclera is derived from the neural crest. In children, it is thinner and shows some of the underlying pigment, appearing slightly blue. In the elderly, fatty deposits on the sclera can make it appear slightly yellow. Many people with dark skin have naturally darkened sclerae, the result of melanin pigmentation.

The spaces of the eye are filled with the aqueous humour anteriorly, between the cornea and lens, and the vitreous body, a jelly-like substance, behind the lens, filling the entire posterior cavity. The aqueous humour is a clear watery fluid that is contained in two areas: the anterior chamber between the cornea and the iris, and the posterior chamber between the iris and the lens. The lens is suspended to the ciliary body by the suspensory ligament (Zonule of Zinn), made up of hundreds of fine transparent fibers which transmit muscular forces to change the shape of the lens for accommodation (focusing). The vitreous body is a clear substance composed of water and proteins, which give it a jelly-like and sticky composition. [6]

Vision

Field of view

Side-view of the human eye, viewed approximately 90deg temporal, illustrating how the iris and pupil appear rotated towards the viewer due to the optical properties of the cornea and the aqueous humor. Mairead cropped.png
Side-view of the human eye, viewed approximately 90° temporal, illustrating how the iris and pupil appear rotated towards the viewer due to the optical properties of the cornea and the aqueous humor.

The approximate field of view of an individual human eye (measured from the fixation point, i.e., the point at which one's gaze is directed) varies by facial anatomy, but is typically 30° superior (up, limited by the brow), 45° nasal (limited by the nose), 70° inferior (down), and 100° temporal (towards the temple). [7] [8] [9] For both eyes combined (binocular) visual field is 135° vertical and 200° horizontal. [10] [11] It is an area of 4.17 steradians or 13700 square degrees for binocular vision. [12] When viewed at large angles from the side, the iris and pupil may still be visible by the viewer, indicating the person has peripheral vision possible at that angle. [13] [14] [15]

About 15° temporal and 1.5° below the horizontal is the blind spot created by the optic nerve nasally, which is roughly 7.5° high and 5.5° wide. [16]

Dynamic range

The retina has a static contrast ratio of around 100:1 (about 6.5 f-stops). As soon as the eye moves rapidly to acquire a target (saccades), it re-adjusts its exposure by adjusting the iris, which adjusts the size of the pupil. Initial dark adaptation takes place in approximately four seconds of profound, uninterrupted darkness; full adaptation through adjustments in retinal rod photoreceptors is 80% complete in thirty minutes. The process is nonlinear and multifaceted, so an interruption by light exposure requires restarting the dark adaptation process over again. Full adaptation is dependent on good blood flow; thus dark adaptation may be hampered by retinal disease, poor vascular circulation and high altitude exposure. [ citation needed ]

The human eye can detect a luminance range of 1014, or one hundred trillion (100,000,000,000,000) (about 46.5 f-stops), from 10−6 cd/m2, or one millionth (0.000001) of a candela per square meter to 108 cd/m2 or one hundred million (100,000,000) candelas per square meter. [17] [18] [19] This range does not include looking at the midday sun (109 cd/m2) [20] or lightning discharge.

At the low end of the range is the absolute threshold of vision for a steady light across a wide field of view, about 10−6 cd/m2 (0.000001 candela per square meter). [21] [22] The upper end of the range is given in terms of normal visual performance as 108 cd/m2 (100,000,000 or one hundred million candelas per square meter). [23]

The eye includes a lens similar to lenses found in optical instruments such as cameras and the same physics principles can be applied. The pupil of the human eye is its aperture; the iris is the diaphragm that serves as the aperture stop. Refraction in the cornea causes the effective aperture (the entrance pupil) to differ slightly from the physical pupil diameter. The entrance pupil is typically about 4 mm in diameter, although it can range from 2 mm (f/8.3) in a brightly lit place to 8 mm (f/2.1) in the dark. The latter value decreases slowly with age; older people's eyes sometimes dilate to not more than 5-6mm in the dark, and may be as small as 1mm in the light. [24] [25]

Eye movement

The light circle is the optic disc where the optic nerve exits the retina Fundus photograph of normal right eye.jpg
The light circle is the optic disc where the optic nerve exits the retina
MRI scan of human eye MRI of human eye.jpg
MRI scan of human eye
Normal anatomy of the human eye and orbit, anterior view Eye orbit anatomy anterior2.jpg
Normal anatomy of the human eye and orbit, anterior view

The visual system in the human brain is too slow to process information if images are slipping across the retina at more than a few degrees per second. [26] Thus, to be able to see while moving, the brain must compensate for the motion of the head by turning the eyes. Frontal-eyed animals have a small area of the retina with very high visual acuity, the fovea centralis. It covers about 2 degrees of visual angle in people. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Any failure to make eye movements correctly can lead to serious visual degradation.

Having two eyes allows the brain to determine the depth and distance of an object, called stereovision, and gives the sense of three-dimensionality to the vision. Both eyes must point accurately enough that the object of regard falls on corresponding points of the two retinas to stimulate stereovision; otherwise, double vision might occur. Some persons with congenitally crossed eyes tend to ignore one eye's vision, thus do not suffer double vision, and do not have stereovision. The movements of the eye are controlled by six muscles attached to each eye, and allow the eye to elevate, depress, converge, diverge and roll. These muscles are both controlled voluntarily and involuntarily to track objects and correct for simultaneous head movements.

Extraocular muscles

Each eye has six muscles that control its movements: the lateral rectus, the medial rectus, the inferior rectus, the superior rectus, the inferior oblique, and the superior oblique. When the muscles exert different tensions, a torque is exerted on the globe that causes it to turn, in almost pure rotation, with only about one millimeter of translation. [27] Thus, the eye can be considered as undergoing rotations about a single point in the center of the eye.

Rapid eye movement

Rapid eye movement, REM, typically refers to the sleep stage during which the most vivid dreams occur. During this stage, the eyes move rapidly.

Saccades

Saccades are quick, simultaneous movements of both eyes in the same direction controlled by the frontal lobe of the brain.

Fixational Eye Movements

Even when looking intently at a single spot, the eyes drift around. This ensures that individual photosensitive cells are continually stimulated in different degrees. Without changing input, these cells would otherwise stop generating output.

A 2019 study [28] showed that small continuous and unconscious eye movements are needed to enable the detection of contrast, and for vision in general. Eye movements include drift, ocular tremor, and microsaccades. Some irregular drifts, movements smaller than a saccade and larger than a microsaccade, subtend up to one tenth of a degree. Researchers vary in their definition of Microsaccades by amplitude, Martin Rolfs [29] states that 'the majority of microsaccades observed in a variety of tasks have amplitudes smaller than 30 min-arc'.

Vestibulo-ocular reflexes

The vestibulo-ocular reflex is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement in response to neural input from the vestibular system of the inner ear, thus maintaining the image in the center of the visual field. For example, when the head moves to the right, the eyes move to the left. This applies for head movements up and down, left and right, and tilt to the right and left, all of which give input to the ocular muscles to maintain visual stability.

Smooth pursuit movement

Eyes can also follow a moving object around. This tracking is less accurate than the vestibulo-ocular reflex, as it requires the brain to process incoming visual information and supply feedback. Following an object moving at constant speed is relatively easy, though the eyes will often make saccadic jerks to keep up. The smooth pursuit movement can move the eye at up to 100°/s in adult humans.

It is more difficult to visually estimate speed in low light conditions or while moving, unless there is another point of reference for determining speed.

Optokinetic reflex

The Optokinetic reflex (or optokinetic nystagmus) stabilizes the image on the retina through visual feedback. It is induced when the entire visual scene drifts across the retina, eliciting eye rotation in the same direction and at a velocity that minimizes the motion of the image on the retina. When the gaze direction deviates too far from the forward heading, a compensatory saccade is induced to reset the gaze to the centre of the visual field. [30]

For example, when looking out of the window at a moving train, the eyes can focus on a moving train for a short moment (by stabilizing it on the retina), until the train moves out of the field of vision. At this point, the eye is moved back to the point where it first saw the train (through a saccade).

Near response

The adjustment to close-range vision involves three processes to focus an image on the retina.

Vergence movement

The two eyes converge to point to the same object. Stereogram Tut Eye Convergence.png
The two eyes converge to point to the same object.

When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both eyes. To look at a nearby object, the eyes rotate 'towards each other' (convergence), while for an object farther away they rotate 'away from each other' (divergence).

Pupil constriction

Lenses cannot refract light rays at their edges as well as they can closer to the center. The image produced by any lens is therefore somewhat blurry around the edges (spherical aberration). It can be minimized by screening out peripheral light rays and looking only at the better-focused center. In the eye, the pupil serves this purpose by constricting while the eye is focused on nearby objects. Small apertures also give an increase in depth of field, allowing a broader range of "in focus" vision. In this way the pupil has a dual purpose for near vision: to reduce spherical aberration and increase depth of field. [31]

Accommodation of the lens

Changing the curvature of the lens is carried out by the ciliary muscles surrounding the lens; this process is called "accommodation". Accommodation narrows the inner diameter of the ciliary body, which actually relaxes the fibers of the suspensory ligament attached to the periphery of the lens, and allows the lens to relax into a more convex, or globular, shape. A more convex lens refracts light more strongly and focuses divergent light rays from near objects onto the retina, allowing closer objects to be brought into better focus. [31] [32]

Clinical significance

Eye care professionals

The human eye contains enough complexity to warrant specialized attention and care beyond the duties of a general practitioner. These specialists, or eye care professionals, serve different functions in different countries. Eye care professionals can have overlap in their patient care privileges. For example, both an ophthalmologist (M.D.) and optometrist (O.D.) are professionals who diagnoses eye disease and can prescribe lenses to correct vision. However, typically only ophthalmologists are licensed to perform surgical procedures. Ophthalmologists may also specialize within a surgical area, such as cornea, cataracts, laser, retina, or oculoplastics. Other eye care professionals include:

Eye irritation

Conjunctival injection, or redness of the sclera surrounding the iris and pupil Bloodshot.jpg
Conjunctival injection, or redness of the sclera surrounding the iris and pupil

Eye irritation has been defined as "the magnitude of any stinging, scratching, burning, or other irritating sensation from the eye". [33] It is a common problem experienced by people of all ages. Related eye symptoms and signs of irritation are discomfort, dryness, excess tearing, itching, grating, foreign body sensation, ocular fatigue, pain, scratchiness, soreness, redness, swollen eyelids, and tiredness, etc. These eye symptoms are reported with intensities from mild to severe. It has been suggested that these eye symptoms are related to different causal mechanisms, and symptoms are related to the particular ocular anatomy involved. [34]

Several suspected causal factors in our environment have been studied so far. [33] One hypothesis is that indoor air pollution may cause eye and airway irritation. [35] [36] Eye irritation depends somewhat on destabilization of the outer-eye tear film, in which the formation of dry spots on the cornea, resulting in ocular discomfort. [35] [37] [38] Occupational factors are also likely to influence the perception of eye irritation. Some of these are lighting (glare and poor contrast), gaze position, reduced blink rate, limited number of breaks from visual tasking, and a constant combination of accommodation, musculoskeletal burden, and impairment of the visual nervous system. [39] [40] Another factor that may be related is work stress. [41] [42] In addition, psychological factors have been found in multivariate analyses to be associated with an increase in eye irritation among VDU users. [43] [44] Other risk factors, such as chemical toxins/irritants (e.g. amines, formaldehyde, acetaldehyde, acrolein, N-decane, VOCs, ozone, pesticides and preservatives, allergens, etc.) might cause eye irritation as well.

Certain volatile organic compounds that are both chemically reactive and airway irritants may cause eye irritation. Personal factors (e.g. use of contact lenses, eye make-up, and certain medications) may also affect destabilization of the tear film and possibly result in more eye symptoms. [34] Nevertheless, if airborne particles alone should destabilize the tear film and cause eye irritation, their content of surface-active compounds must be high. [34] An integrated physiological risk model with blink frequency, destabilization, and break-up of the eye tear film as inseparable phenomena may explain eye irritation among office workers in terms of occupational, climate, and eye-related physiological risk factors. [34]

There are two major measures of eye irritation. One is blink frequency which can be observed by human behavior. The other measures are break up time, tear flow, hyperemia (redness, swelling), tear fluid cytology, and epithelial damage (vital stains) etc., which are human beings' physiological reactions. Blink frequency is defined as the number of blinks per minute and it is associated with eye irritation. Blink frequencies are individual with mean frequencies of < 2-3 to 20-30 blinks/minute, and they depend on environmental factors including the use of contact lenses. Dehydration, mental activities, work conditions, room temperature, relative humidity, and illumination all influence blink frequency. Break-up time (BUT) is another major measure of eye irritation and tear film stability. [45] It is defined as the time interval (in seconds) between blinking and rupture. BUT is considered to reflect the stability of the tear film as well. In normal persons, the break-up time exceeds the interval between blinks, and, therefore, the tear film is maintained. [34] Studies have shown that blink frequency is correlated negatively with break-up time. This phenomenon indicates that perceived eye irritation is associated with an increase in blink frequency since the cornea and conjunctiva both have sensitive nerve endings that belong to the first trigeminal branch. [46] [47] Other evaluating methods, such as hyperemia, cytology etc. have increasingly been used to assess eye irritation.

There are other factors that are related to eye irritation as well. Three major factors that influence the most are indoor air pollution, contact lenses and gender differences. Field studies have found that the prevalence of objective eye signs is often significantly altered among office workers in comparisons with random samples of the general population. [48] [49] [50] [51] These research results might indicate that indoor air pollution has played an important role in causing eye irritation. There are more and more people wearing contact lens now and dry eyes appear to be the most common complaint among contact lens wearers. [52] [53] [54] Although both contact lens wearers and spectacle wearers experience similar eye irritation symptoms, dryness, redness, and grittiness have been reported far more frequently among contact lens wearers and with greater severity than among spectacle wearers. [54] Studies have shown that incidence of dry eyes increases with age, [55] [56] especially among women. [57] Tear film stability (e.g. break-up time) is significantly lower among women than among men. In addition, women have a higher blink frequency while reading. [58] Several factors may contribute to gender differences. One is the use of eye make-up. Another reason could be that the women in the reported studies have done more VDU work than the men, including lower grade work. A third often-quoted explanation is related to the age-dependent decrease of tear secretion, particularly among women after 40 years of age. [57] [59] [60]

In a study conducted by UCLA, the frequency of reported symptoms in industrial buildings was investigated. [61] The study's results were that eye irritation was the most frequent symptom in industrial building spaces, at 81%. Modern office work with use of office equipment has raised concerns about possible adverse health effects. [62] Since the 1970s, reports have linked mucosal, skin, and general symptoms to work with self-copying paper. Emission of various particulate and volatile substances has been suggested as specific causes. These symptoms have been related to sick building syndrome (SBS), which involves symptoms such as irritation to the eyes, skin, and upper airways, headache and fatigue. [63]

Many of the symptoms described in SBS and multiple chemical sensitivity (MCS) resemble the symptoms known to be elicited by airborne irritant chemicals. [64] A repeated measurement design was employed in the study of acute symptoms of eye and respiratory tract irritation resulting from occupational exposure to sodium borate dusts. [65] The symptom assessment of the 79 exposed and 27 unexposed subjects comprised interviews before the shift began and then at regular hourly intervals for the next six hours of the shift, four days in a row. [65] Exposures were monitored concurrently with a personal real time aerosol monitor. Two different exposure profiles, a daily average and short term (15 minute) average, were used in the analysis. Exposure-response relations were evaluated by linking incidence rates for each symptom with categories of exposure. [65]

Acute incidence rates for nasal, eye, and throat irritation, and coughing and breathlessness were found to be associated with increased exposure levels of both exposure indices. Steeper exposure-response slopes were seen when short term exposure concentrations were used. Results from multivariate logistic regression analysis suggest that current smokers tended to be less sensitive to the exposure to airborne sodium borate dust. [65]

Several actions can be taken to prevent eye irritation—

In addition, other measures are proper lid hygiene, avoidance of eye rubbing, [73] and proper use of personal products and medication. Eye make-up should be used with care. [74]

Eye disease

Diagram of a human eye (horizontal section of the right eye)
1. Lens, 2. Zonule of Zinn or Ciliary zonule, 3. Posterior chamber and 4. Anterior chamber with 5. Aqueous humour flow; 6. Pupil, 7. Corneosclera or Fibrous tunic with 8. Cornea, 9. Trabecular meshwork and Schlemm's canal. 10. Corneal limbus and 11. Sclera; 12. Conjunctiva, 13. Uvea with 14. Iris, 15. Ciliary body (with a: pars plicata and b: pars plana) and 16. Choroid); 17. Ora serrata, 18. Vitreous humor with 19. Hyaloid canal/(old artery), 20. Retina with 21. Macula or macula lutea, 22. Fovea and 23. Optic disc - blind spot; 24. Optical axis of the eye. 25. Axis of eye, 26. Optic nerve with 27. Dural sheath, 28. Tenon's capsule or bulbar sheath, 29. Tendon.
30. Anterior segment, 31. Posterior segment.
32. Ophthalmic artery, 33. Artery and central retinal vein - 36. Blood vessels of the retina; Ciliary arteries (34. Short posterior ones, 35. Long posterior ones and 37. Anterior ones), 38. Lacrimal artery, 39. Ophthalmic vein, 40. Vorticose vein.
41. Ethmoid bone, 42. Medial rectus muscle, 43. Lateral rectus muscle, 44. Sphenoid bone. Schematic diagram of human eye multilingual.svg
Diagram of a human eye (horizontal section of the right eye)
1. Lens, 2. Zonule of Zinn or Ciliary zonule, 3. Posterior chamber and 4. Anterior chamber with 5. Aqueous humour flow; 6. Pupil, 7. Corneosclera or Fibrous tunic with 8. Cornea, 9. Trabecular meshwork and Schlemm's canal. 10. Corneal limbus and 11. Sclera; 12. Conjunctiva, 13. Uvea with 14. Iris, 15. Ciliary body (with a: pars plicata and b: pars plana) and 16. Choroid); 17. Ora serrata, 18. Vitreous humor with 19. Hyaloid canal/(old artery), 20. Retina with 21. Macula or macula lutea, 22. Fovea and 23. Optic discblind spot; 24. Optical axis of the eye. 25. Axis of eye, 26. Optic nerve with 27. Dural sheath, 28. Tenon's capsule or bulbar sheath, 29. Tendon.
30. Anterior segment, 31. Posterior segment.
32. Ophthalmic artery, 33. Artery and central retinal vein → 36. Blood vessels of the retina; Ciliary arteries (34. Short posterior ones, 35. Long posterior ones and 37. Anterior ones), 38. Lacrimal artery, 39. Ophthalmic vein, 40. Vorticose vein.
41. Ethmoid bone, 42. Medial rectus muscle, 43. Lateral rectus muscle, 44. Sphenoid bone.

There are many diseases, disorders, and age-related changes that may affect the eyes and surrounding structures.

As the eye ages, certain changes occur that can be attributed solely to the aging process. Most of these anatomic and physiologic processes follow a gradual decline. With aging, the quality of vision worsens due to reasons independent of diseases of the aging eye. While there are many changes of significance in the non-diseased eye, the most functionally important changes seem to be a reduction in pupil size and the loss of accommodation or focusing capability (presbyopia). The area of the pupil governs the amount of light that can reach the retina. The extent to which the pupil dilates decreases with age, leading to a substantial decrease in light received at the retina. In comparison to younger people, it is as though older persons are constantly wearing medium-density sunglasses. Therefore, for any detailed visually guided tasks on which performance varies with illumination, older persons require extra lighting. Certain ocular diseases can come from sexually transmitted diseases such as herpes and genital warts. If contact between the eye and area of infection occurs, the STD can be transmitted to the eye. [75]

With aging, a prominent white ring develops in the periphery of the cornea called arcus senilis. Aging causes laxity, downward shift of eyelid tissues and atrophy of the orbital fat. These changes contribute to the etiology of several eyelid disorders such as ectropion, entropion, dermatochalasis, and ptosis. The vitreous gel undergoes liquefaction (posterior vitreous detachment or PVD) and its opacities — visible as floaters — gradually increase in number.

Various eye care professionals, including ophthalmologists (eye doctors/surgeons), optometrists, and opticians, are involved in the treatment and management of ocular and vision disorders. A Snellen chart is one type of eye chart used to measure visual acuity. At the conclusion of a complete eye examination, the eye doctor might provide the patient with an eyeglass prescription for corrective lenses. Some disorders of the eyes for which corrective lenses are prescribed include myopia (near-sightedness) which affects about one-third[ citation needed ] of the human population, hyperopia (far-sightedness) which affects about one quarter of the population, astigmatism, and presbyopia (the loss of focusing range during aging).

Macular degeneration

Macular degeneration is especially prevalent in the U.S. and affects roughly 1.75 million Americans each year. [76] Having lower levels of lutein and zeaxanthin within the macula may be associated with an increase in the risk of age-related macular degeneration. [77] < Lutein and zeaxanthin act as antioxidants that protect the retina and macula from oxidative damage from high-energy light waves. [78] As the light waves enter the eye they excite electrons that can cause harm to the cells in the eye, but before they can cause oxidative damage that may lead to macular degeneration or cataracts. Lutein and zeaxanthin bind to the electron free radical and are reduced rendering the electron safe. There are many ways to ensure a diet rich in lutein and zeaxanthin, the best of which is to eat dark green vegetables including kale, spinach, broccoli and turnip greens. Nutrition is an important aspect of the ability to achieve and maintain proper eye health. Lutein and zeaxanthin are two major carotenoids, found in the macula of the eye, that are being researched to identify their role in the pathogenesis of eye disorders such as age-related macular degeneration and cataracts. [79]

Additional images

See also

Related Research Articles

Ophthalmology field of medicine treating eye disorders

Ophthalmology is a branch of medicine and surgery that deals with the diagnosis and treatment of eye disorders. An ophthalmologist is a specialist in ophthalmology. Their credentials include a degree in medicine, followed by an additional four years of ophthalmology residency training. They may or may not receive residency training in internal medicine, pediatrics, or general surgery before the ophthalmology residency. Additional training may be sought through a fellowship in a particular specialty of eye pathology. Ophthalmologists are allowed to use medications to treat eye diseases, implement laser therapy, and perform surgery when needed. Ophthalmologists may participate in academic research on the diagnosis and treatment for eye disorders.

Near-sightedness problem with distance vision

Near-sightedness, also known as short-sightedness and myopia, is an eye disorder where light focuses in front of, instead of on, the retina. This causes distant objects to be blurry while close objects appear normal. Other symptoms may include headaches and eye strain. Severe near-sightedness increases the risk of retinal detachment, cataracts, and glaucoma.

Cataract clouding of the lens inside the eye, which leads to low vision

A cataract is a clouding of the lens in the eye which leads to a decrease in vision. Cataracts often develop slowly and can affect one or both eyes. Symptoms may include faded colors, blurry vision, halos around light, trouble with bright lights, and trouble seeing at night. This may result in trouble driving, reading, or recognizing faces. Poor vision caused by cataracts may also result in an increased risk of falling and depression. Cataracts cause half of all cases of blindness and 33% of visual impairment worldwide.

Eye organ that detects light and converts it into electro-chemical impulses in neurons

Eyes are organs of the visual system. They provide organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and convert it into electro-chemical impulses in neurons. In higher organisms, the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system. Image-resolving eyes are present in molluscs, chordates and arthropods.

Macula of retina

The macula or macula lutea is an oval-shaped pigmented area near the center of the retina of the human eye and some other animalian eyes. The macula in humans has a diameter of around 5.5 mm (0.22 in) and is subdivided into the umbo, foveola, foveal avascular zone, fovea, parafovea, and perifovea areas.

Keratoconus corneal disease characterized by structural changes within the cornea causing it to thin and change, leading to a protruding conical shape

Keratoconus (KC) is a disorder of the eye which results in progressive thinning of the cornea. This may result in blurry vision, double vision, nearsightedness, astigmatism, and light sensitivity. Usually both eyes are affected. In more severe cases a scarring or a circle may be seen within the cornea.

Far-sightedness human eye system

Far-sightedness, also known as hyperopia, is a condition of the eye in which light is focused behind, instead of on, the retina. This results in close objects appearing blurry, while far objects may appear normal. As the condition worsens, objects at all distances may be blurry. Other symptoms may include headaches and eye strain. People may also experience accommodative dysfunction, binocular dysfunction, amblyopia, and strabismus.

Photophobia is a symptom of abnormal intolerance to visual perception of light. As a medical symptom, photophobia is not a morbid fear or phobia, but an experience of discomfort or pain to the eyes due to light exposure or by presence of actual physical sensitivity of the eyes, though the term is sometimes additionally applied to abnormal or irrational fear of light such as heliophobia. The term photophobia comes from the Greek φῶς (phōs), meaning "light", and φόβος (phóbos), meaning "fear". Photophobia is a common symptom of visual snow.

Radial keratotomy

Radial keratotomy (RK) is a refractive surgical procedure to correct myopia (nearsightedness) that was developed in 1974, by Svyatoslav Fyodorov, a Russian ophthalmologist. It has been largely supplanted by newer operations, such as photorefractive keratectomy, LASIK, Epi-LASIK and the phakic intraocular lens.

Dry eye syndrome condition of having dry eyes

Dry eye syndrome (DES), also known as keratoconjunctivitis sicca (KCS), is the condition of having dry eyes. Other associated symptoms include irritation, redness, discharge, and easily fatigued eyes. Blurred vision may also occur. The symptoms can range from mild and occasional to severe and continuous. Scarring of the cornea may occur in some cases without treatment.

Eye examination a series of tests assessing vision and pertaining to the eyes

An eye examination is a series of tests performed by an ophthalmologist, optometrist, or orthoptist, optician (UK), assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes. Health care professionals often recommend that all people should have periodic and thorough eye examinations as part of routine primary care, especially since many eye diseases are asymptomatic.

Red eye (medicine) type of eye disease

A red eye is an eye that appears red due to illness or injury. It is usually injection and prominence of the superficial blood vessels of the conjunctiva, which may be caused by disorders of these or adjacent structures. Conjunctivitis and subconjunctival hemorrhage are two of the less serious but more common causes.

Computer vision syndrome (CVS) is a condition resulting from focusing the eyes on a computer or other display device for protracted, uninterrupted periods of time and the eye muscles being unable to recover from the strain due to a lack of adequate sleep. Some symptoms of CVS include headaches, blurred vision, neck pain, fatigue, eye strain, dry eyes, irritated eyes, double vision, vertigo/dizziness, polyopia, and difficulty refocusing the eyes. These symptoms can be further aggravated by improper lighting conditions or air moving past the eyes.

Lutein chemical compound

Lutein is a xanthophyll and one of 600 known naturally occurring carotenoids. Lutein is synthesized only by plants and like other xanthophylls is found in high quantities in green leafy vegetables such as spinach, kale and yellow carrots. In green plants, xanthophylls act to modulate light energy and serve as non-photochemical quenching agents to deal with triplet chlorophyll, which is overproduced at very high light levels, during photosynthesis. See xanthophyll cycle for this topic.

Zeaxanthin chemical compound

Zeaxanthin is one of the most common carotenoid alcohols found in nature. It is important in the xanthophyll cycle. Synthesized in plants and some micro-organisms, it is the pigment that gives paprika, corn, saffron, wolfberries, and many other plants and microbes their characteristic color.

Emmetropia is the state of vision where a faraway object at infinity is in sharp focus with the eye lens in a neutral or relaxed state. This condition of the normal eye is achieved when the refractive power of the cornea and the axial length of the eye balance out, which focuses rays exactly on the retina, resulting in perfect vision. A human eye in a state of emmetropia requires no corrective lenses; the vision scores well on a visual acuity test. For example, on a Snellen chart test, emmetropic eyes score at "6/6"(m) or "20/20"(ft) vision, meaning that at a distance of 20 ft they see as well as a normal eye at a distance of 20 ft. Either myopic (near-sighted) eyes or hyperopic (far-sighted) eyes would score worse, e.g. 20/40. A person with exceptionally acute vision might score at 20/15.

The eye is made up of the sclera, the iris, and the pupil, a black hole located at the center of the eye with the main function of allowing light to pass to the retina. Due to certain muscle spasms in the eye, the pupil can resemble a tadpole, which consists of a circular body, no arms or legs, and a tail.

<i>Meso</i>-zeaxanthin chemical compound

Meso-zeaxanthin (3R,3´S-zeaxanthin) is a xanthophyll carotenoid, as it contains oxygen and hydrocarbons, and is one of the three stereoisomers of zeaxanthin. Of the three stereoisomers, meso-zeaxanthin is the second most abundant in nature after 3R,3´R-zeaxanthin, which is produced by plants and algae. To date, meso-zeaxanthin has been identified in specific tissues of marine organisms and in the macula lutea, also known as the "yellow spot", of the human retina.

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